(1) Technical Field of the Invention
This invention relates to an air intake system of a multicylinder internal combustion engine.
(2) Prior Art
Some of conventional multicylinder internal combustion engines include an intake system equipped with an intake passage having resonance portions for the two rows of the engine cylinders and a throttle valve unit installed near the outside or upper portion of one of the two cylinder rows due to the layout restriction depending on the type of the engine or the limitations of vehicle installation. The intake passage therefore consists of resonance portions which differ in length.
For instance, a V-type multicylinder internal combustion engine 1 with two banks, shown in the FIG. 2, includes two rows of cylinders, one row having cylinders #1, #3, #6 and the other having cylinders #2, #4, #6. Combustion does not take place sequentially in the three cylinders of the same row. The cylinders are connected via surge tanks 4, 5 to a throttle valve unit 6 provided near the outside or top portion of the cylinder row 2 in the engine 1. The throttle valve unit 6 has two internal paths where twin throttle valves 7, 8 are provided, respectively. The valves are capable of performing synchronous rotation, that is, the vavles can rotate together at the same time and in the same degree.
An intake passage 11 extending from the unit 6 to the air cleaner 9, has a separating wall which divides the pipe's internal passage into two separate sections.
In the case of said intake passage 11, even if the cross-sectional areas S.sub.1, S.sub.2 of resonance portions 13, 14 extending from the wall end 12 fo the separating wall 10 to the surge tanks 4, 5, respectively are made equal, it is inevitable to set the length L.sub.1 of resonance portion 13 connected to the cylinder row 2 to a length smaller than the length L.sub.2 of resonance portion 14 connected to the other cylinder row 3, because the throttle valve unit 6 is provided on the side of the cylinders of the row 2 in the engine.
FIG. 3 shows the volumetric efficiency .eta. relative to the engine speed N of the engine 1 with the resonance pipes 13, 14 of different length. As obviously shown, the peak volumetric efficiency .eta. in lower engine speed range as a result of resonance supercharging effect is separated into two peaks; the P.sub.1 by the cylinder row 2 and the P.sub.2 by the row 3. The deviation of intake air distribution between the two cylinder rows can lead to variations of generating torque from the cylinders between the two rows, increased engine vibrations and a poor feeling of vehicle driving.
Especially in gasoline engines, the control by calculation of fuel flow rate based on the total air flow rate in the engine as a whole does not serve to ensure a constant fuel-air ratio for each cylinder, because of the difference in volumetric efficiency between the cylinders. This results in an increase of harmful components in exhaust gases, in addition to the above mentioned variations of generating torque.
The peak volumetric efficiency P.sub.3 in the characteristic curve supercharging, shown in FIG. 3 corresponds to the occurrence of inertia in high engine speed, of the inlet passage 21 which connects the surge tank to each cylinder.
Japanese patent unexamined publication No. 55-19976 discloses a proposed intake system for elimination of the deviated distribution of intake air over two rows of engine cylinders. It includes resonance passage portions of different length connected to the two cylinder rows. These portions have different sectional areas for correcting the length difference to attain resonance speed synchronism. However, the volumetric efficiency when inertia supercharging takes place does not show the same value between the two cylinder rows. Therefore, the proposed system does not provide a satisfactory result to eliminate the problem of deviated air distribution.